Hot melt thermoplastic adhesives have been widely used in industry for adhering many types of products, and are particularly useful in applications where quick setting time is advantageous. One application for hot melt adhesive which has been of considerable interest in recent years is the bonding of non-woven fibrous material to a polyurethane substrate in articles such as disposable diapers, incontinence pads and similar articles.
One aspect of forming an appropriate bond between the non-woven layer and polyurethane substrate of a disposable diaper, for example, is to avoid loss of adhesive in the valleys or gaps formed in the irregular surface of the chopped fibrous or fluff-type material which forms the non-woven layer. If the adhesive is discharged onto the non-woven layer in droplet form, for example, a portion of the droplets can fall between the gaps in the surface of the fibrous, non-woven material. As a result, additional quantities of adhesive are required to obtain the desired bond strength between the polyurethane substrate and non-woven material.
This problem has been overcome in the prior art by forming hot melt thermoplastic adhesives in elongated, thin beads or fibers which are deposited atop the non-woven material and span the gaps in its irregular surface. Elongated beads or fibers of adhesive have been produced in prior art spray devices which include a nozzle formed with an adhesive discharge opening and one or more air jet orifices through which a jet of air is ejected. A bead of adhesive is ejected from the adhesive discharge opening in the nozzle which is then impinged by the air jets to attenuate or stretch the adhesive bead forming a thin fiber for deposition onto the substrate. Examples of spray devices of this type are disclosed in U.S. Pat. Nos. 2,626,424 to Hawthorne, Jr.; 3,152,923 to Marshall et al; and, 4,185,981 to Ohsato et al.
In applications such as the formation of disposable diapers, it is important to carefully control the spray pattern of adhesive fibers deposited onto the non-woven substrate in order to obtain the desired bond strength between the non-woven layer and polyurethane substrates using as little adhesive as possible. Improved control of the spray pattern of adhesive fibers has been obtained in prior art spray devices of the type described above by directing the air jets which impact the adhesive bead discharged from the nozzle substantially tangent to the outer periphery of the adhesive bead. The tangentially directed air jets rotate the elongated fibers of adhesive formed from the adhesive bead ejected from the adhesive discharge opening in the gun nozzle in a relatively tight, compact spiral pattern for application onto the substrate. Structure which produces a spiral spray pattern of adhesive fibers for deposition onto a substrate is disclosed, for example, in the '424 Hawthorne, Jr. patent and '981 Ohsato et al patent mentioned above.
In order to obtain a spray pattern upon a substrate which is wider than that produced by a single gun, while at the same time controlling the location of the pattern or the substrate, two or more spray guns each effective to produce a separate spiral spray pattern are required. To avoid gaps in the adhesive layer applied to the substrate, and/or prevent overlapping of adjacent spray patterns which could result in an unwanted buildup of adhesive, the separate spiral spray patterns from adjacent guns are preferably tangent to one another at or near the surface of the substrate. This has presented a problem in the prior art wherein adjacent rotating spiral spray patterns form eddies or turbulence in the area where they contact one another. This turbulence disrupts the spray pattern near the surface of the substrate and also tends to lift at least some of the attenuated adhesive fibers back upstream toward the spray guns where they adhere to the equipment. It also creates gaps in the spray pattern.